Of the various apparatuses for continously casting a thin metallic strip, there is known one incorporating a twin roll type mold. As shown in FIG. 9, a type twin roll type mold comprises a pair of rolls 41 adapted to be driven at a constant speed by unillustrated rotary drive means, and a melt receiver 43 provided above the rolls 41 and consisting of four rectangularly arranged lateral walls 42 (only three of which are shown). Molten metal A supplied into the melt receiver 43 solidifies and is drawn out as a casting B by the rototing rolls 41 to produce a thin metallic strip.
In the casting apparatus of the above construction, however, the withdrawal at a constant speed of the casting B leads to the following problem.
The molten metal A solidifies upon contact with the outer surface of each internally cooled roll 41 to continuously form a casting shell C as shown in FIG. 10. The temperature of the shell C is higher on the molten metal side than on the roll side, so that the shell C is subjected to a deforming force due to different degrees of contraction within the shell. This results in local reduction in contact force between the shell C and the roll 41 and in an extreme case leads to actual deformation of the shell C with resultant formation of gaps a as shown in FIG. 11. Once such a state occurs, subsequent growth of the shell C causes irregularities in the thickness thereof involving projections and depressions as shown in FIG. 12, and the shell projections upon passage through the outlet clearance of the mold come into contact with the projections of another similarly produced shell C on the other roll 41 to sealingly trap the molten metal therebelow. The thus trapped molten metal subsequently solidifies with a volmetric reduction to accelerate unevenness in the thickness of the casting B or otherwise result in the formation of internal voids, consequently deteriorating the quality of the product.
On the other hand, as illustrated in FIG. 13, a restraining shell D also grows on a corresponding lateral wall 42 adjacent each roll 41 during the casting operation to ultimately merge at its thin leading edge with the thin trailing edge of the casting shell C. The restraining shell D tends to restrain the forward movement of the casting shell C while the latter is forcibly advanced by the continuous rotation of the roll 41, so that the merged shells C, D are immediately torn apart at the thin connection therebetween. The separated restraining shell D again grows shortly thereafter and rejoins with the casting shell C to repeat the same tearing process. As a result, a cut is formed on each side surface of the casting B (FIGS. 9 and 12) every time the shells C, D are torn apart, the cut being the cause of subsequent break out.
The above adverse conditions (for deformation and for tear mark formation) will usually continue as long as the rolls 41 are rotated at a constant speed for constant speed withdrawal of the casting.